Yeah - I adjusted this one every direction while I had the hole open and it was still under pressure, but it wouldn't drain. I poked a drill bit in the drain hole, thinking it might be limed up, and after getting it apart, I can see that it needed to be forced down more so the top would clear the drain hole, but I think something would have busted if I adjusted it any tighter. It needed a full 1/8" farther down to clear the drain hole. The rubber stop was deformed/swelled - maybe from old age?? Most of my experience with rubber would find it getting hard and brittle as it aged, but this thing was still soft.
I've got three more just like it that won't drain. When it quits raining I'm gonna tighten one of them down until it either drains, or busts something, ha.
Thanks again for the advice.